As a vibration damping device, there has been known a variable damping force damper using a Magneto-Rheological Fluid (MRF) composition whose apparent viscosity varies according to intensity of a magnetic field applied thereto. In this vibration damping device, input vibration is damped by flow resistance of the MRF composition when a piston reciprocates in a cylinder enclosing the MRF composition. With this vibration damping device, it is possible to adjust a damping force of vibration depending on strength of an applied magnetic field.
Incidentally, a composition in which magnetic particles are dispersed in a dispersion medium is generally used as the MRF composition. Therefore, in the vibration damping device using the MRF composition, wear of a frictional sliding portion due to presence of the magnetic particles is a problem.
A sliding portion between a piston rod and a rod guide in the vibration damping device using a conventional MRF composition is a dry friction structure in which a rod guide bearing sliding portion is provided outside the MRF liquid chamber sealed by an oil seal. Thus, this vibration damping device avoids excessive wear under an environment in which the magnetic particles are present as described above.
However, since a conventional rod guide sliding structure is a dry friction, frictional force between the piston rod and the rod guide is high, and it is easily influenced by temperature and humidity and is poor in stability. Further, the conventional rod guide sliding structure has a disadvantage that dependency on operation speed and lateral force load received by the damper is increased. High friction characteristics specific to the vibration damping device using such an MRF composition is a major technical problem in improving ride comfort performance and application to a lightweight vehicle.
In order to solve this problem, friction wear has ever been reduced by improving the rod guide sliding structure. Specifically, a vibration damping device is disclosed in which a piston rod connected to a piston is slidably supported by a bearing member at an end portion of a cylinder, and the bearing member is surrounded by a porous member (for example, see Patent Document 1). With this vibration damping device, since only liquid component of the MRF composition whose magnetic particles are filtered by the porous member are supplied to the frictional sliding portion between the piston rod and the bearing member, the magnetic particles are prevented from intervening in the frictional sliding portion.